In optical transmission systems wherein light energy propagates along an optical transmission path, a need often arises for coupling a portion of the optical energy to various system components that are located between the source of the optical energy and a remote terminal apparatus. For example, in an optical communications system, information -- transmitted as optical energy -- is carried to a remote receiver unit via one or more optical transmission lines such as optical fibers, and it is often necessary to couple the information to a plurality of individual communications units or stations located along the path of the optical transmission line.
In the prior art various apparatus for coupling a portion of the energy propagating along such optical transmission paths have been proposed. One such prior art coupler, which is relevant to this invention, is commonly identified as an optical Tee coupler. In prior art optical Tee couplers, a primary transmission path is formed by two sections of optical transmission line such as optical fibers or glass rods with the end faces of the optical transmission lines being spaced apart from one another and the longitudinal axis of the two sections being colinear. A third section of optical transmission line is positioned relative to the first and second sections to form the vertical member of a Tee-like configuration. More explicitly, the longitudinal axis of the third section of optical transmission line is coplanar with the longitudinal axes of the first and second sections and intersects the common axis of the two sections at a point between the opposing end faces of the first and second sections of optical transmission line. The end face of the third section of transmission line is spaced outwardly from the edge boundaries of the first and second transmission line sections and a beam splitter is disposed between the end faces of three sections of optical transmission line. The beam splitter divides the optical energy propagating along the first section of transmission line into two separate optical signals, with one signal passing through the beam splitter into the second section of transmission line and one signal being directed into the third section of transmission line. Thus, in effect the third section of transmission line forms a secondary transmission path to provide a portion or sample of the energy flowing through a primary transmission path that includes the first and second sections of transmission line.
The beam splitter of prior art Tee couplers are generally a plane-parallel sheet of transparent material such as glass having a semi-reflective coating on one surface, or two transparent right prisms joined together to form a cube with a partially reflecting interface disposed at a 45.degree. angle between two opposite edges of the cube. In any case, the beam splitter is arranged to form a semi-reflective layer that passes through the intersection of the longitudinal axis of the three sections of transmission line and is orthogonal to the coplanar sections of transmission line. The semi-reflective layer of such prior art optical Tee coupler can be a partially reflective mirrored surface or a sheet of transparent material having a lower refractive index than that of the material surrounding the semi-reflective sheet (e.g., air, glass rods or prisms used to transmit the optical energy between the end faces of the transmission line sections and the surface of the beam splitter, or a fluid or gel known as index matching fluid that has a predetermined refractive index). As optical energy propagates from the end face of the first section of transmission line, the energy travels to the semi-reflective layer, a portion of the optical energy passes through the layer and propagates into the face of the second section of transmission line, and a portion of the optical energy is reflected from the partially reflective layer into the end face of the third optical transmission line.
Although such prior art Tee couplers have proven satisfactory in applications wherein optical energy is transmitted primarily for illuminating a spatial region at the terminus of each optical path, such arrangements have several drawbacks in apparatus such as optical communication systems wherein minimum perturbation of the light propagating through the communication system and the minimum loss of optical energy is of prime importance. With respect to the perturbation of the optical energy in prior art optical Tee couplers, both the optical energy propagating through the coupler into the primary transmission line and that portion of the optical energy coupled from the primary transmission path into the secondary transmission path undergo undesirable perturbations. One cause of such perturbations that applies to both the optical energy reflected into the secondary transmission path and the optical energy propagating through the coupler into the primary transmission path arises because the refractive index of prior art beam splitters is a function of the frequency of the optical energy. Since both the angle of reflection and the angle at which the unreflected optical energy is refracted while traveling through the beam splitter are dependent on the refractive index of the beam splitter material, it can be recognized that each frequency component of the optical energy is subjected to varying amounts of reflection and refraction. This characteristic of prior art couplers is especially detrimental in multi-mode optical transmission systems wherein the optical signal of interest includes a number of frequency components within a particular band or region of the optical spectrum. Further, since the optical path through the beam splitter of the prior art couplers is relatively long with respect to the wavelength of the optical energy and such prior art beam splitters include at least two surfaces at which reflections occur, each ray of light impinging on the beam splitter and propagating therethrough can be reflected several times. If energy from all such reflections reaches the secondary transmission path, the optical energy coupled from the third section of transmission line will not faithfully correspond to the signal propagating into the first section of transmission line. That is, considering a single ray of optical energy propagating from the first section of transmission line and impinging on a prior art beam splitter, such as a relatively thick planar-parallel glass sheet, it can be recognized that a portion of such energy is reflected at the first surface of the glass sheet and a portion of the energy transmitted through the glass sheet is also reflected as the energy propagates from the beam splitter toward the second section of transmission line. Since both reflections will be directed toward the secondary optical transmission path, in effect, the secondary path receives two separate signals representing the same information, but separated from one another in both time and space. This phenomena results in a condition commonly referred to as "ghost images" and can greatly increase the problems associated with extracting the information contained in the optical energy that propagates along the third section of transmission line.
The refraction of the optical energy traveling through the beam splitter also causes problems with respect to the optical energy transmitted through the beam splitter and into the second transmission line, i.e. the optical energy flowing through the primary transmission path. In this respect, since the refractive index of the prior art beam splitters is generally lower than the refractive index of the optical paths (or of the material surrounding the beam splitter), a beam of light energy effectively diverges as it passes through the beam splitter causing refractive errors in the optical signal propagating along the primary transmission line. Although there have been attempts within the prior art to compensate for the above-described perturbations of the optical signals, these attempts have generally comprised the addition of optical elements which complicate the coupler structure without providing totally satisfactory results.
With respect to the efficiency of prior art couplers, it can be recognized that, because of the relatively thick prior art beam splitter and other geometric considerations, the end faces of the optical transmission lines are separated from one another by a relatively large distance. As is known in the art, when optical energy is radiated from the end face of an optical transmission line such as a glass rod or optical fiber, the energy forms a diverging beam having a divergence angle proportional to the numerical aperture of the optical transmission line. Such divergence effectively causes a decrease in the energy density of the optical waves propagating through the beam splitter and toward the end face of the second section of transmission line. Hence, unless the cross-sectional area of the second transmission line is substantially larger than the cross-sectional area of the first transmission line to thereby intercept essentially all of the optical energy, or unless other means are employed to at least partially eliminate the divergence, considerable energy loss will occur within a prior art coupler.
In view of the above-described geometric considerations, it can be recognized that problems which effect undesirable perturbations in the optical energy and undesirable decreases in coupler efficiency become more acute in couplers necessarily employing optical transmission lines of relatively small cross-sectional area. In this respect, present day circular multimode optical fibers commonly have a diameter on the order of 0.003 to 0.005 inches while single mode optical fibers can have a diameter less than 10 microns. Accordingly, much of the prior art, although suitable for use with relatively large optical transmission lines such as glass rods, is not suited for use in systems employing such miniature optical fibers. Additionally, in many instances an optical transmission system includes a relatively large number of individual optical fibers which are arranged within a fiberoptic cable with each optical fiber carrying separate information. In such installations, it is often necessary to couple a portion of the optical energy propagating through each of the individual fibers into associated optical transmission lines. Since prior art optical Tee couplers for coupling a portion of the optical energy flowing through a single optical transmission line are relatively large, it can be realized that a large number of such prior art couplers for coupling energy from each optical transmission line is not a desirable approach to such a problem.
Accordingly, it is an object of this invention to provide optical coupling apparatus of improved efficiency wherein minimum perturbation occurs in the optical energy propagating through the coupling apparatus.
Further, it is an object of this invention to provide a relatively small optical coupler of simple construction wherein optical energy is coupled from each optical path of a fiberoptic cable.